Field of the Invention:
The invention is directed to a multipiece control spool for a spool valve in which spool members are coupled together by a hook arrangement that allows for limited axial misalignment.
Spool valves are widely used in hydraulic control systems to control the flow of hydraulic fluid. The elongated control spool is positioned inside a valve body having an elongated cavity for accepting the spool. The valve body is provided with ports through which hydraulic fluid can communicate with the elongated cavity. The control spool is provided with control surfaces which control the flow of hydraulic fluid into and out of the cavity. The hydraulic fluid flow pattern is varied by adjusting the axial insertion distance of the spool into the cavity. The spool may also be provided with detents which cooperate with a spring biased ball mounted in the valve body for defining specific axial locations of the spool.
As hydraulic control systems become more sophisticated, the system designer increasingly wants to include more hydraulic control functions in each valve. As the number of hydraulic functions increase, the length of the spool and the valve housing must also increase. However, practically, because of the close tolerances involved, there is an upper limit as to the length of a spool and the length of the housing a manufacturer can properly produce. One way to lengthen spools has been to make multipiece spools that are provided with hooks which couple adjoining spool members together to form a longer control spool. Such a spool configuration is disclosed in U. S. Pat. Nos. 4,418,711 and 4,576,203.
Although longer spools can be fabricated in the manner suggested above, the spool manufacturer must still make the valve body cavity to very close tolerances to accept the extended spool. Typically, to maintain these close tolerances, the valve body cavity is bored from only one end so that the cavity is coaxial throughout. Therefore, the longer the cavity must be bored, the more complicated the machining. As such, it is desirable to bore the valve cavity from both ends to minimize the length of the boring operations. However, such an operation, unless it is very carefully monitored, can result in a non-coaxial cavity. The present invention discloses a control spool having means for providing limited axial misalignment between the spool members and as such provides for larger allowable tolerances in the valve body thereby reducing manufacturing costs.